In a conventional line scan circuit, such as in the Plessey or Mullard TBA920 integrated circuit, a phase-locked oscillator is synchronized with the incoming horizontal synchronization (H-sync) pulses to provide a control signal to the line output circuit. This ensures that oscillation is maintained in the absence of the H-sync signal, for example, during maintenance in the case of a display terminal, or, in the case of a TV receiver, when untuned, and noise on the incoming signal is smoothed out. The phase relationship between the output and input is very precisely maintained to prevent line tearing and other picture defects.
Flyback voltage pulses occur at the terminals of the yoke of the cathode ray tube (CRT) between successive horizontal scans of the electron beam while the screen is blanked. Due to component tolerances and temperature effects, the flyback voltage pulses may be broadened and unless the broadened pulse falls within the blanking period, picture defects can occur. Conventionally, this problem has been overcome by designing the CRT circuits to allow a broad blanking period between the line scans: however, this has a deleterious effect on the efficiency of the display.